In the Equation $R \times p _ { k } = M P K + \Delta p _ { k } , \Delta p _ { k }$

In the equation $R \times p _ { k } = M P K + \Delta p _ { k } , \Delta p _ { k }$ is the:

A) arbitrage equation
B) real interest rate
C) rate of return to capital
D) marginal product of capital
E) opportunity cost of capital